In a liquid crystal display (LCD), a backlight control circuit is used which controls LEDs to illuminate from the back side of an LCD screen, so that a user can observe an image from the front side of the LCD screen.
FIG. 1 shows a conventional backlight control circuit with LEDs all connected in parallel. As shown in the figure, in a backlight control circuit 20, the currents passing through LEDs L1-LN are respectively controlled by the current sources CS1-CSN. The backlight control circuit 20 comprises a lowest voltage selection circuit 21 which chooses a lowest voltage among all voltages at cathode ends of the LEDs L1-LN, and an error amplifier circuit 13 compares the lowest voltage with a reference voltage to generate a signal controlling the voltage supply circuit 11. By feedback control mechanism, the one with the lowest voltage among the nodes N11-N1N is kept at the voltage level of the reference voltage Vref; thus, the output voltage Vout is under control so that all current source circuits are provided with sufficient operating voltage for normal operation, and all LEDs can illuminate normally thereby. Moreover, to prevent the voltage supply circuit 11 from unlimitedly increasing the output voltage Vout (for example, when the error amplifier circuit 13 malfunctions), an over voltage protection circuit (OVP Ckt.) 12 is provided in the backlight control circuit 20, which detects the output voltage Vout and sends a signal to stop the voltage supply circuit 11 from increasing the output voltage Vout if the output voltage Vout is excessively high. (Depending on circuit design, the voltage supply can be totally stopped, or kept at an upper limit value. The latter is more popular in a backlight control circuit.)
FIG. 2 shows a typical structure of an over voltage protection circuit 12, wherein the output voltage Vout is monitored by comparing the voltage at the node Vsense2 with a reference voltage Vovp. The result of comparison determines a signal for controlling the voltage supply circuit 11.
Because the number of LEDs that are allowed to be connected all in parallel in the above conventional arrangement is limited, it naturally leads to connecting the LEDs partially in series and partially in parallel (series-parallel connection). FIG. 3 shows a series-parallel connection circuit for LEDs, which employs the backlight control circuit 20 shown in FIG. 1 and providing equal number of LEDs in each of the paths 101-10N.
In the conventional arrangements described above, the feedback control mechanism is designed to keep the one with the lowest current among the LED paths 101-10N above a predetermined value; ideally, this predetermined value should be the minimum current required for the LEDs in every path to operate normally. In practice, this is done by keeping the one with the lowest voltage among the nodes N11-N1N at the voltage level of the reference voltage Vref. However, between different LEDs, the voltage drops may be different because of deviations in manufacture. Hence, to ensure that every LED in every path operates normally, a circuit designer usually determines the value of the reference voltage Vref in a conservative manner; that is, the manually predetermined reference voltage Vref is usually higher than what is actually required (the optimum, lowest voltage) for the circuit. Accordingly, the output voltage Vout is unnecessarily increased, causing unnecessary power consumption.